This invention relates to a method and apparatus for electro-optical distance measurement.
In the prior art, various types of electro-optical distance measuring apparatuses using the propagation velocity of light in space or in air are well known. Most broadly used are instruments with sinusoidal modulation of the brightness of a light beam. After having travelled twice the measured distance up to an optical reflector and back again, said modulation of the light beam undergoes a phase shift which is related to the distance and which is measured by optical and electric means. After recent progress in the development of electric time measurement techniques, the time of propagation of single light impulses or flashes also has been measured once or repetitively for determining distance.
Until now, less frequently used are instruments for distance measurement according to the so called tooth-wheel method (A.H.L. Fizeau, 1846). Originally, this method consisted of periodically interrupting a beam of light by means of a tooth wheel, transmitting the interrupted beam to a reflector and after retro-reflection periodically interrupting said beam a second time by the same tooth-wheel. Due to its retardation, the light beam, with a convenient number of revolutions per minute of the tooth-wheel, on its return will hit a tooth instead of a gap and be thus blocked from observation. With this method, the time of travel of the beam is calculated from the number of revolutions for the above mentioned case. According to the state of the art, electro-optical crystals are used instead of the tooth-wheel (see U.S. Pat. No. 3,424,531 to P. L. Bender et al.). Such crystals, instead of interruptions, produce a periodic modulation of elliptical polarization of the light beam. A linearly polarized beam with suitable orientation of its plane of polarization with respect to the axes of the electro-optic crystal is modulated with a sinusoidal electric signal of some 100 MHz. If retro-reflected beam components upon their second pass through the crystal in reverse direction meet the same phase of modulation as on their first pass, the original steady state linear polarization is restored and complete darkness of those beam components is observed behind a suitable optical analyzer. This is the case when at each moment the total number of modulation wavelengths present over twice the measured distance from the crystal to the retro-reflector and back is an integer number. If it is not, the brightness of the beam will not be minimum, but then a minimum may be obtained by changing the measured distance or the wavelength of modulation. Both methods are state of the art (see also GB Pat. No. 919,368 to K. D. Froome et al.).
Various known instruments for electro-optical distance measurement comprising electro-optic crystal modulators are made to cooperate with one or more corner-cube reflectors as target means. These corner-cubes retro-reflect an incident measuring beam essentailly on its original path back to the distance measuring instrument. The original path also includes the modulating crystal such that an almost complete demodulation is possible and the above mentioned minimum of brightness may be observed. The completeness of this retro-reflection depends on manufacturing precision of the corner-cube reflectors, which must be rather high and also must be maintained during the measurement procedure. Such reflectors consequently have a rather high price and are delicate and difficult to handle.
If one tries to aim a distance measuring instrument with an electro-optic crystal modulator of known type on to an arbitrary unprepared object or on a reflecting foil instead of a corner-cube reflector for measurement, none of the above mentioned brightness minima will occur and no measurement is possible, although sufficiently intensive radiation components are received from the targeted object.